Question with INVO sample application - understanding Redirection

I've posted a few questions about redirection that have occured in small applications I have been creating using Phalcon. I've been looking at the INVO sample application and am confused by the behaviour if I alter some code. In SessionController, startAction, on a successful login, the code forwards to 'invoices/index' but the browser address does not reflect this change. If I replace the forward with a redirect like:

I'm using latest Phalcon on the latest MAMP. Just to be clear, I understand why Phalcon results in an error. I just don't understand why doing an external redirect results in the funky browser address/url of "http://localhost:8888/invo/session/invoices/index"?

Is this the correct, expected behaviour and if so, why? I've pasted the altered code below. Any comments much appreciate! :)

@quasipickle - thanks very much for your comments. It seemed odd to see the "confused" browser address value and I wondered whether there was in fact an explanation for it. What you are saying sounds reasonable :) Cheers.